If I understand it properly the 'clean up' is done on fields that are treated as Dimensions. I'm not sure why this wouldn't happen when you refresh the data-source. Do you have an example of one of the fields in your data-set? (eg: "Country_Name" changed to "Country Name"). Have you selected the option for Data Interpreter?
On a side note you may be interested in Tableau Prep (a great tool for data cleanup)
my data-source is an Oracle database, not Excel.
An example of a field "clean-up" is:
DESTINATION_DELIVERY_LOCATION => Destination Delivery Location
I'm not too familiar with using Oracle databases, so I'm not sure what could be going wrong. I'll continue to look for more info on this for you. Meanwhile can you keep us posted if you manage to find a fix? (there might be something on other sites like the Oracle community or Reddit)